1. Field of the Invention
The present invention relates to air mass sensors, in particular hot-film air-mass sensors, which include an auxiliary heater for preventing contamination of a sensor area. The present invention also relates to measures for checking the proper functioning of the auxiliary heater of an air mass sensor.
2. Description of the Related Art
An air mass sensor is used in an engine system of a motor vehicle to measure a mass flow rate of fresh air drawn in by an internal combustion engine. In air-guided internal combustion engines, the ascertained air mass flow rate is provided as an input variable for a calculation of a fuel quantity to be injected.
An air mass sensor designed as a hot-film air-mass sensor (HFM sensor) includes a heated sensor area, which is situated in an air flow. Cooling effectuated by the air flow is compensated for by provided heat output, so that the sensor area is held at a constant temperature. The heat output required for this depends on the air mass flow rate of the air flow. As an alternative, cooling effectuated by the air flow may be ascertained via the sensor diaphragm and a corresponding temperature difference may be provided as information about the air mass flow rate.
An auxiliary heater may be provided in the air mass sensor in order to keep the sensor area free from contamination. Such an air mass sensor including a heatable sensor area is described in the published German patent document DE 101 11 840 C2. An auxiliary heater is provided, which may be operated in such a way that thermogradient eddies are formed in the area of the auxiliary heater, which result in precipitates of contamination in the air flow in the area of the auxiliary heater outside of the sensor area. The contamination may result, for example, from the ambient air or from oil vapors introduced into the air supply system from a crankcase when the engine is shut off.
Failure of the auxiliary heater results in a slow contamination of the sensor area, which may result in a progressive degradation of the measurement and ultimately in unusable measured values.